Impulse analysis for flow sensor-based fluid control system

ABSTRACT

A fluid flow control system using flow rates to extract additional information from an in-line flow sensor. The system provides the ability to determine a position of a movable flow sensor element of a flow sensor by illuminating a photosensitive pixel array with a light source to create a first set of pixel intensity values introducing an abrupt change to the fluid driving pressure, illuminating the photosensitive pixel array with a light source to create a second set of pixel intensity values, and calculating the difference between the first and second sets of pixel intensity values as a function of pixel position.

BACKGROUND

The present disclosure relates to fluid flow control systems, such asintravenous infusion pumps, and more particularly to feedback controlinfusion pumps with flow sensing, volume sensing, variable pressurecontrol, and variable flow resistance. In particular, the presentdisclosure relates to a method and apparatus for extracting enhancedinformation from an in-line fluid flow sensor by imposing an abrupt flowrate change.

A conventional large volume infusion pump is typically equipped with amotor that, in connection with a mechanical assembly and through theinterface of a fluid barrier, pushes a small amount of fluid per motor“step.” The mechanism might be a cam, a leadscrew, or other suchassembly. The fluid barrier might be a syringe, an extruded tube, amolded cassette, or other such device that separates the pumpingmechanism from the fluid in question. In each case, the fluid movementis determined by a certain number of motor steps over time.

At slow flow rates, the motor steps are relatively infrequent with longdwell periods. At high flow rates, the motor and mechanism are run attheir maximal capacity until one element has reached its engineeringlimit. The flow rate is inherently pulsatile, yet this pulsatile natureis less significant at higher flow rates where the natural compliance ofthe outlet of the pumps serves to dampen the pulses into more or less acontinuous stream of fluid.

The motors used conventionally are inherently powerful enough toovercome significant forces and resistances, so they are capable ofgenerating significant pumping forces. This forceful pumping is anartifact and has no desirable clinical effect. The sensing mechanismscommonly used are pressure based and are made with indirect contact withthe fluid to be pumped. In most cases, the fluid barrier, such as anextruded tube, exerts far more force than the internal fluid pressures.The result is a lack of sensitivity to pressure changes and a lack offeedback as to the actual conditions of fluid flow. It is common forconventional pumps to operate indefinitely without recognizing that theactual fluid flow rate is far below the targeted level.

Conventional motor driven pumps are notoriously inefficient with respectto external power consumption. For devices that have a high requirementfor portability, this power inefficiency translates into unreliableoperation.

Prior to the use of pumps, most infusions were done by the adjustment ofa gravity-based pressure (e.g., by adjusting the height of a liquidcontainer) and the adjustment of inline resistance (e.g., by moving theposition of a roller clamp), both in response to an inline flow sensingmethod (e.g., performed by a user counting drops into an air chamber).Although this prior art method was labor intensive and had a limitedrate range, it offered some significant advantages over the subsequent“advances” in technology. First, the use of gravity head heights for adelivery pressure was energy efficient. No external power supply wasrequired. Second, the pressure was low, so the dangers of high-pressureinfusions were avoided. Third, the gravity infusions could be augmentedwith a low cost and readily available pressure cuff, supplementing thefluid flow to rates well above those possible by an instrumented “pump”line. Forth, a gravity administration was not capable of infusing largeamounts of air into the output line, because the hydrostatic pressuregoes to zero as the fluid source empties.

An ideal infusion system will combine the meritorious aspects of aconventional “gravity” infusion with the benefits of a controlledintravenous infusion pump. In each aspect, this disclosure takes thedesired principles of a gravity infusion and reduces the dependence uponskilled labor and extends the range and precision of fluid flow controland provides advanced information management capabilities.

An ideal embodiment of an infusion device would be one with continuousflow, wide flow rate range, high energy efficiency, accuracy of volumedelivered over time, minimal operating pressures, maximum sensitivity toexternal conditions, freedom from false alarms for air-in-line,simplicity, low cost, intuitive operation, automated informationexchange, safety, and reliability.

Certain infusions have historically been managed by air pressuredelivery systems, most commonly found in the operating room and inemergency situations. Prior art attempts have been made to determine theflow rate via pressure monitoring and control. For example, U.S. Pat.No. 5,207,645 to Ross et al. discloses pressurizing an IV bag andmonitoring pressure to infer flow rates. However, the prior art systemslack independent flow sensing, and, therefore, do not offer enoughinformation to provide accurate and safe infusions.

Under the best of circumstances, there is not enough information in thepressure signal alone to provide the accuracy needed for intravenousinfusion therapy. Furthermore, there are a number of likely failuremodes that would go undetected using the pressure signal alone. Aninfusion pump must be able to respond to events in a relevant timeframe. International standards suggest that a maximum period of 20seconds can lapse before fluid delivery is considered “non-continuous.”As an example, for an infusion of 10 ml/h, the system would want toresolve 20 seconds of flow, which corresponds to 0.056 mL. This volumerepresents one part in 18,000 of air volume of a 1,000 mL bladder.Temperature induced change in pressure brought about by a normal airconditioning cycle is far greater than this signal. The measurement ofpressure alone is not adequate for an intravenous infusion device. Nogeneral purpose, full range, infusion devices using pressure-controlleddelivery are known to be on the market.

An entire class of “passive” infusion pumps exists whereby a constantpressure is created on a fluid filled container by way of a spring,elastomeric structure, gas producing chemical equilibrium, or othermeans. This constant pressure fluid is fed into a high resistance outputline, providing relatively stable fluid flow.

In typical pressure based flow control products, a relatively highpressure pushes fluid into a known, high, and fixed resistance,providing a constant flow rate with good immunity from changes inexternal conditions. It is a purpose of our prior commonly ownedprovisional application Ser. No. 60/777,193, filed on Feb. 27, 2006, andPCT Publication Nos. WO2007/098287, WO2007/098265, and WO2007/106232 toprovide a highly flexible flow control system with a very broad flowrate range, operating under minimal pressures, with a relatively low andvariable resistance. The entire contents of the aforementionedprovisional and PCT applications are incorporated herein by reference.

Embodiments of such devices control fluid flow based on a responsivefluid flow sensing means that forms a closed loop control by changingboth the fluid driving pressure and the inline resistance. In contrastto the conventional approach to flow control wherein a user observesfluid flowing as it formed drops in an air chamber, then adjustspressure by varying the head height of the fluid source, and thenadjusts the inline resistance via a manual valve, our above-mentioneddisclosures employ a flow sensing apparatus and method thatautomatically and accurately measures fluid flow rate, precisely adjuststhe hydrostatic pressure of the fluid source, and precisely adjustsinline fluid flow resistance to achieve or maintain a target flow rate.

SUMMARY

Certain embodiments of an in-line fluid flow sensor are based on theposition of an object in the flow path in which the force of the fluidflow is balanced by an opposing force. The resultant equilibriumposition is a function of the speed of fluid flow and of the fluidviscosity being measured.

The present disclosure describes an apparatus and method of enhancingthe information derived from such an in-line fluid flow sensor byexamining its response to an abrupt change in flow rate. The response ofthe fluid flow sensor can enhance the sensitivity of the measurement,may provide diagnostic value, and can provide additional information,such as fluid viscosity.

In certain embodiments of a fluid flow sensor, a signal can be analyzedto determine the position of an object in the flow path. This signal mayhave complex characteristics and the feature extraction that indicatesthe ball position may be challenged by the complexity of the signal. Ifa flow rate change is imposed upon the system, then the difference inthe flow sensor signals taken at different flow rates will eliminatemuch of the underlying complexity and provide simplified featureextraction methods. In this way, the sensitivity of the flow sensor isenhanced, even with complex features or noisy environments.

At any given point in time, the flow sensor signal represents a certainflow rate. When a flow rate change is imposed by a change in fluiddriving pressure, then the resultant response is an indication of totalsystemic fluid flow resistance. In an infusion control device, thismeasurement of fluid flow resistance can have significant clinical anddiagnostic value.

In certain embodiments of a fluid flow sensor, the viscosity of thefluid will represent an offset in the position of a flow object. Takenby itself, however, the flow sensor may not be able to distinguishbetween a change in fluid viscosity and changes in flow sensor responsedue to normal manufacturing tolerances. If a flow rate change isabruptly applied, then the difference in flow sensor response in thesensor can be used to infer fluid viscosity, because the underlyingoffset due to manufacturing tolerances is eliminated from the analysis.The absolute change in flow object position as a function of flow ratechange is an indication of fluid viscosity. In a further analysis, thespeed with which the flow object moves to its new equilibrium positionis an additional function of fluid viscosity and may be based on eitheror both of an absolute position change and rate of position change ofthe sensor object.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments and are notto be construed as limiting the invention.

FIG. 1 is a functional block diagram of a fluid pumping system operableto embody an exemplary embodiment of the present invention.

FIG. 2 is a functional block diagram flow sensor and control circuit forthe system appearing in FIG. 1.

FIG. 3 is an isometric sectional view illustrating an exemplary flowsensor with integral resistor element.

FIGS. 4A and 4B are isometric views of an exemplary flow sensor withintegral resistor showing the emitter and receiver.

FIG. 4C is a side view of the flow sensor embodiment appearing in FIGS.4A and 4B.

FIG. 5 is a graph of pixel voltage output for photosensor array fordetermining flow sensor element location.

FIG. 6 is a graph of pixel voltage output for photosensor array withseparate plots for flow sensor element location before and after achange in ball sensor element position.

FIG. 7 is a graph of pixel voltage differences for the plots appearingin FIG. 6.

FIG. 8 is a flow diagram outlining an exemplary method for determiningsensor element location in accordance with the present invention.

FIG. 9 is graph of flow sensor element position as a function of timeduring a period of an abrupt change in flow rate.

FIG. 10 is a flow diagram outlining an exemplary embodiment for sensingfluid viscosity in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 depicts an exemplary flow controlsystem 100 in accordance with an exemplary embodiment of the presentinvention. The system includes a pressure frame 10 that is of knowntotal volume and contains within it an air bladder 20 and a flexible bag30 that contains within it a liquid 40 to be delivered.

The air bladder 20 is connected to a charging tank 60 of known volumevia a conduit or line 22 extending between an outlet of the tank 60 andan inlet of the bladder 20. A pneumatic pump 50 is pneumatically coupledto an inlet of the charging tank 60 via a line 52. A bladder valve 24 inthe line 22 may be selectively opened and closed to selectively coupleand decouple the outlet of the tank 60 with the inlet of the bladder 20.The charging tank may selectively be vented to atmosphere via a tankvent valve 62. The air bladder 20 may be vented to atmosphere via anoptional bladder vent valve (not shown). Alternatively, the bladder 20may be vented to atmosphere by opening the valves 24 and 62.

The tank 60 is connected to a tank pressure sensor 66 and a tanktemperature sensor 68. The bladder 20 is connected to a bladder pressuresensor 26 and a bladder temperature sensor 28.

The liquid 40 is fluidically coupled to an output 70 via an inline flowsensor 80, a fluid flow resistor 90, and an output line 72. The liquid40 may be, for example, a medication fluid, intravenous solution, bloodproduct, or the like, to be infused and the output 70 may be, forexample, a patient or subject in need thereof. In the depictedembodiment of FIG. 1, the flow resistor 90 is shown downstream of thein-line flow sensor 80. Alternatively, the flow resistor 90 may bepositioned upstream of the flow sensor 80. The flow resistor 90 and flowsensor 80 may be separate or may be integrally formed.

In reference to FIG. 2, an embodiment of the fluid control system 100includes the pump 50 including a pump motor 51, the bladder valve 24including a bladder valve motor 25, the tank vent valve 62 including atank vent valve motor 63, the flow sensor 80 including an optical sensor824 and an optical emitter 822, the flow resistor 90 including a flowresistor motor 91, the tank pressure sensor 66, tank temperature sensor68, bladder pressure sensor 26, bladder temperature sensor 28, a sensorprocessor 210, a controller processor 212, a pump motor controller 214,a tank vent valve motor controller 216, a bladder valve motor controller218 and a flow resistor motor controller 220.

The sensor processor 210, controller processor 212, pump motorcontroller 214, tank vent valve motor controller 216, bladder valvemotor controller 218, and flow resistor motor controller 220 may beimplemented in a microprocessor, microcontroller, controller, embeddedcontroller, or the like. Although the processors 210 and 212 and thecontrollers 214-220 are depicted in FIG. 2 as discrete modules orprocessors for conceptual simplicity and ease of exposition, it is to beappreciated that modules 210-214 can share common hardware. Well-knowninternal components for processing and control modules, such as powersupplies, analog-to-digital converters, clock circuitry, etc., are notshown in FIG. 2 for simplicity and would be understood by personsskilled in the art.

The controller processor 212 controls the pump 50 via the pump motorcontroller 214, the tank vent valve 62 via the tank vent valvecontroller 216, the bladder valve 24 via the bladder valve controller218, and the flow resistor 90 via the flow resistor motor controller220. Alternatively, the controller processor 212 may control one or moreof the motors directly or via any other suitable known device. Thecontroller 212 may also control the application of power to the opticalemitter 822.

The sensor processor 210 receives a signal indicative of bladdertemperature and pressure from the bladder temperature sensor 28 andbladder pressure sensor 26, respectively. The sensor processor 210receives a signal indicative of tank temperature and pressure from thetank temperature sensor 68 and tank pressure sensor 66, respectively.The sensor processor 210 receives a signal from the optical sensor 824indicative of the position of a flow sensor indicator element in theflow path as described below.

FIG. 3 shows an exemplary flow sensor 80 with integral flow resistor 90.The flow resistor 90 includes an inlet end 910 fluidly coupled to thefluid source 40 and an outlet 912 fluidly coupled to an inlet 810 of theflow sensor 80. The flow sensor 80 includes an outlet end 812 fluidlycoupled to the output 50 such as the vasculature of a patient, e.g., viaan IV catheter or cannula as generally known in the art. Although theinline sensor 80 and flow restrictor 90 are depicted as an integralassembly in the embodiment of FIGS. 3 and 4A-4C, it will be recognizedthat the flow resistor and the flow sensor units may be discreteassemblies fluidically coupled in serial fashion.

In reference to FIGS. 3 and 4A-4C, the flow resistor 90 includes arotatable housing 914, which may have a plurality of radially extendingribs or projections 916 forming a gear that may be selectively rotatedby the motor 91, which may be a stepper motor having an intermeshingmember, or the like. The rotatable housing 914 is coupled to an axiallymovable needle resistor 917 wherein rotating the housing 914 in onedirection causes the needle resistor 917 to move in one axial directionand rotating the housing 914 in the opposite direction causes the needleresistor 917 to move in the opposite axial direction, for example, viahelical threads formed on an interior surface of the rotatable housingmember 914. As best seen in FIG. 3, the needle resistor axially movesbetween a first, closed position wherein the needle resistor engages amating seat 918 and a fully open position. An annular gap 920 definedbetween the needle resistor 917 and the seat 918 increases as the valvemoves from the closed position to the fully open position, therebyproviding a variable flow resistance, which varies as a function of thedegree of rotation of the housing 914.

The flow sensor 80 includes a housing portion 814 defining an axialchannel or bore 816 receiving a ball member 818. A spring member 820urges the ball member 818 in a direction opposite to the direction offlow. The spring member 820 may be a coil spring (e.g., conical orcylindrical coil spring) or may be another resiliently compressiblematerial such as a foam member, deflectable band or leaf spring, or thelike.

The spring 820 bears against the ball 818 and applies a force to theball in the direction opposite to the direction of fluid flow. Anadjustment mechanism, such as a threaded member engaging the fixed endof the spring 820 may be provided to axially advance or retract thefixed spring end to adjust the force preload of the spring 820 on theball 818. In operation, fluid flow will exert a force on the sensor ball818 against the urging of the spring 820, which force increases as theflow rate increases. The ball 818 thus moves until an equilibriumposition is reached such that the force of the compression spring 820 onthe ball 818 is balanced by the force of the fluid flow against the ball818.

In reference to FIGS. 4A-4C, the optical emitter 822, which may be, forexample, an LED array, is provided on a first side of the housing 814and the optical receiver 824, which may be, a photosensitive array,charge-coupled device (CCD) array, photodiode array, complimentary metaloxide semiconductor (CMOS) digital detector array, or the like, isprovided on a second side of the housing 814 opposite the first side.The optical emitter 822 transmits light through the housing 814 and intothe cavity 816. The light incident upon the ball 818 is transmittedthrough the ball 818 and opposite wall of the housing 814 to form alight intensity pattern on the optical sensor 824. Where the fluidflowing through the channel 816 is a generally opaque fluid or otherwisehas a high absorbance of the light emitted by the emitter 822, the ball818 may be a clear ball, e.g., formed of acrylic or other transparentpolymeric material, which serves to dramatically reduce the optical pathlength of the fluid in the optical path between the emitter 822 and thesensor 824 in the vicinity of the ball 818, thereby reducing theabsorption of light by the fluid surrounding the ball in the flowpassageway. Also, the use of a clear ball sensor element 818 allows theball to function as a lens to transmit and focus the light.

The optical transmitter 822 may include one or more light sourceelements having a wavelength, for example, in the infrared (IR),visible, or ultraviolet (UV) region and the housing and ball member maybe formed of a material that optically transmits light of the lightsource wavelength. The light source 822 may be an array of lightelements, such as LEDs, or laser, etc. The light source may be segmentedalong the axis or may be a continuous, e.g., scanned or otherwiseoptically formed beam. The light source may illuminate the detectorarray along its length simultaneously or by sequentially scanning alongits length. The refractive effect of a transparent ball member may havea focusing effect on the light passing therethrough that may be detectedby the photosensor array. Alternatively, a nontransmissive ball 818 maybe employed and the ball position may be determined by detecting theposition of a shadow cast by the ball on the photosensor array. In stillfurther embodiments, the ball member may have reflective surface and theoptical sensor array may be positioned to detect light reflected fromthe surface of said ball.

The output from the photosensitive array is a set of pixel voltagevalues which vary in accordance with the amount of light impinging onthe each pixel of the photosensitive array. The pixel voltage values maybe sampled and digitized using an analog-to-digital converter and storedas digital data in an electronic storage medium as a numericalrepresentation of the pixel output voltage levels, and thus, lightintensity levels, along the detector array.

The output of the optical sensor 824 may be passed to the sensorprocessor 210, which may include a position-detection module orcircuitry wherein the axial position of the ball 818 within the channel816 is determined. The axial position of the ball 816 may in turn beused to determine a flow rate and/or calibrate or correlate ballpositions with known flow rates calculated by other means such as pluralvolume measurements made using the method outlined in the aforementionedU.S. provisional application Ser. No. 60 and PCT Publication Nos.WO2007/098287, WO2007/098265, or WO2007/106232.

Referring now to FIG. 5, there appears a graph of pixel voltage signal230 of the photosensor array 824 as a function of pixel position. In thedepicted example, the pixel voltage measurements were made usinghalf-&-half as the fluid 40 and the flow sensor 80 was specificallydetuned to represent a worst case scenario for the flow sensor andprovide maximum challenge to the fluid control system. The graph of FIG.5 shows that the flow sensor signal is complex and difficult to analyzefor the position 231 of the flow object, which is somewhat ambiguous.

Referring now to FIG. 6, the ball 820 was moved by the imposition of amodified flow rate and a subsequent measurement of the pixel voltagevalues of the photosensor array 824 was made (see signal 232). The newball position 233, based on the pixel voltage values, is likewisesomewhat ambiguous.

FIG. 7 is a graph 234 of the pixel voltage differences between the firstsignal 230 and the second signal 232. Subtracting the second signal fromthe first signal cancels or reduces common mode complexity and/or noiseof the two signals and the first ball position 231 a and second ballposition 233 a appear as clearly identifiable peaks, even thoughpositions 231 and 233 based on the individual signals 230 and 232,respectively, were ambiguous. Alternatively, the first signal can besubtracted from the second signal, in which case the ball position canbe similarly determined, but wherein the resultant function will be thenegative function relative to the function 234 appearing in FIG. 7,i.e., reflected about the x-axis.

A method for detecting the flow sensor indicator element is outlined inthe flowchart of FIG. 8. At step 240, a first signal from thephotodetector array is provided to the sensor processor 210. At step244, the flow rate is changed. The flow rate may be changed byintroducing air into the charging tank 60 with the pump 50 to increasethe pressure in the tank to a pressure greater than the pressure in thebladder 20 and opening the bladder valve 24. The pressure increase inthe bladder 20 is preferably an abrupt pressure increase, e.g., toprovide a step function change in fluid driving pressure, e.g., bypopping or otherwise rapidly opening the valve 24. Alternatively, thechange in flow rate may be a decrease in pressure. For example, if thepressure in the charging tank 60 is lower than the pressure in thebladder 20, then the rapid opening of the valve 24 will abruptly reducethe driving pressure. In alternative embodiments, an optional bladdervent valve (not shown) may be provided for venting the bladder to reducethe pressure in the bladder 20.

At step 248, a second signal from the photodetector array is provided tothe sensor processor 210 representative of fluid flow rate at the newdriving pressure. At step 252, one the first and second signals issubtracted from the other to provide clearly identifiable peaksrepresentative of the ball axial position as described above.

It will be recognized that in a flow control system employing apressurized bladder as the fluid driving force, it may be necessary toperiodically increase the pressure in the bladder, for example, toachieve a desired flow rate. Also, once a desired flow rate has beenachieved, periodic increases in the bladder 20 pressure will benecessary to maintain a desired flow rate since the bladder 20 willexpand and the pressure in the bladder 20, and thus flow rate willthereby decay, as the fluid 40 exits the bag 30 and is delivered to thesubject 70. Thus, even where the primary purpose of the pressureincrease in the bladder 20 is to establish or maintain a desired flowrate, the observation of the ball position using the sensor 824 beforeand after the pressure increase in accordance with the presentdisclosure provides an additional benefit in that ball position can bedetermined with enhanced accuracy.

As discussed above, comparing ball position before and after an abruptchange in flow rate can advantageously be used to provide a clearindication of sensor ball position. In a further aspect, observation ofball position during the abrupt change in flow rate provides the abilityto measure viscosity of the fluid 40. It has been found that viscosityof the fluid 40 can be determined by one or both of (1) the distance theball moves in response to a change in flow rate (driving pressure); aswell as (2) the rate at which the ball moves to the new position. Thehigher the viscosity, the further the ball moves in response to a changein flow rate. In addition, the higher the viscosity of the fluid, thelonger it takes for the ball to assume its new equilibrium position inresponse to an abrupt change in flow rate.

Referring now to FIG. 9, there appears a graph in which there is plotteda curve 260 representative of sensor ball position as a function of timeduring an abrupt increase in fluid driving pressure for a low viscosityfluid. The ball moved from position 2 to position 11, for a span of nineunits of difference. The slope 262 represents the speed at which thesensor ball moved from its initial position to its final position forthe low viscosity fluid. A curve 264 is representative of sensor ballposition as a function of time for the same change in flow rate for arelatively high viscosity fluid. The ball moved from position 0 toposition 13, for a span of 13 units of difference with the highviscosity fluid. The slope 266 represents the speed at which the sensorball moved from its initial position to its final position for the highviscosity fluid.

The slope 266 for the high viscosity fluid is lower than the slope 262for the low viscosity fluid, and the distance moved for the higherviscosity was greater than the distance moved for the lower viscosityfluid, thus indicating that, for higher viscosities, the fluid will pushthe ball further, yet will do so at a lower speed taking significantlylonger to reach its equilibrium position. The graph also shows how thenominal starting positions 268 and 270 for the low and high viscosityfluids, respectively, for the same flow rates may vary due to thedifference in viscosity.

Referring now to FIG. 10, a method for determining the viscosity of afluid being delivered in a flow control system is illustrated. At step280, an abrupt change in flow rate is effected, e.g., by increasing thepressure in the tank 60 and popping the valve 24 to introduce a stepchange in fluid driving pressure. At step 284, the axial position of thesensor ball is monitored as a function of time during the flow ratechange until the ball assumes a new equilibrium position. Alternatively,the change in flow rate may be effected by reducing the pressure in thebladder 20, e.g., by reducing the pressure in the tank 60 and poppingthe valve 24, or, by using an optional bladder vent valve (not shown).

At step 288, one or both of absolute position change and the rate ofposition change of the flow sensor element is calculated, e.g., bycomparing ball pixel position along the sensor array and/or bydetermining the average slope of position as a function of time for theperiod of time in which it took the ball to move from its initialequilibrium position at the initial flow rate to its new equilibrium atthe new flow rate. At step 292, the viscosity of the fluid beingdelivered is determined from the change in ball position and/or rate ofsensor element response, for example, by comparing calculated ballposition change and/or rate thereof to prestored values for fluids ofknown viscosity, which may be stored in database, look up table, datafile, etc.

In operation, the type of fluid 40 to be infused may be input into theflow control system, e.g., by the operator using a user interface of theprocessor 210 and/or 212. Alternatively, the type of fluid 40 may beidentified by reading a bar code (or other optically readable indicia)or radio frequency identification (RFID) tag on or in the fluidcontainer, e.g., by a bar code (optical) scanner or RFID scanner. Theviscosity as determined in step 292 may then be checked to determinewhether it is consistent with an expected fluid viscosity based onprestored viscosity characteristics associated with the fluid type inputby the operator (e.g., stored in a database, lookup table, data file,memory, etc.). For example, in the case of IV infusion fluids, manyfluids or at least categories of fluids, such as blood products (e.g.,whole blood, platelets, plasma, immunoglobulins, packed red cells etc.),saline, dextrose, albumin, lactated ringers solution, amino acids, lipidemulsions, parenteral nutritional solutions, etc., will have differentviscosity characteristics. If the viscosity determined at step 292 isdifferent from the expected viscosity, the operator may be alerted tothis potential error condition, thereby providing an additionalsafeguard.

In further aspects, the observation of ball movement during an abruptchange fluid driving pressure may also be used to detect other errorconditions. The change in flow rate in response to a change in fluiddriving pressure is indicative of the total systemic resistance. Forexample, if the ball position does not change after the fluid drivingpressure is increased, the line may be occluded, and the operator may bealerted to this potential error condition. Additionally, in the face ofa potential occlusion, the pressure in the bladder may be reduced to alower level, e.g., using a bladder vent valve, if provided, or byreducing the pressure in the charging tank 60 (e.g., via tank vent valve62) and opening the bladder valve 24, e.g., to reduce the chance of anunwanted release bolus.

The invention has been described with reference to the preferredembodiments. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A method for determining a position of a movable flow sensor elementof a flow sensor in a flow control system, the flow sensor being of atype having an axially-extending flow passageway for a liquid to bedelivered by the flow control system, the movable sensor element beingaxially movable within the flow passageway in response to a first forceexerted on the sensor element by the liquid as it passes through theflow passageway and in response to a second force exerted on the sensorelement in a direction opposite the first force by a spring receivedwithin the flow passageway, the sensor reaching equilibrium positionwithin the flow passageway when the magnitudes of the first and secondforces are equal, the equilibrium position corresponding to a axialposition within the flow passageway which varies with varying flow rate,the flow control system being of a type having a source of fluid drivingpressure for driving the fluid through the flow passageway, the flowrate being responsive to variations in the fluid driving pressure, themethod comprising: illuminating a photosensitive pixel array with alight source, the photosensitive pixel array extending axially along theflow passageway and the sensor element disposed in an optical pathbetween the light source and the photosensitive pixel array to generatea first set of pixel intensity values for the photosensitive pixelarray, the first set of pixel intensity values representative of a firstsensor element position corresponding to a first flow rate; introducingan abrupt change in the fluid driving pressure; illuminating thephotosensitive pixel array with the light source to generate a secondset of pixel intensity values for the photosensitive pixel array, thesecond set of pixel intensity values representative of a second sensorelement position corresponding to a second flow rate; calculating adifference between the first and second sets of pixel intensity valuesas a function of pixel position.
 2. The method of claim 1, wherein thefirst and second sets of pixel intensity values are valuesrepresentative of pixel voltages.
 3. The method of claim 1, wherein thesensor element is an optically transmissive ball.
 4. The method of claim1, further comprising: identifying a first pixel position correspondingto a maxima of the difference between the first and second sets of pixelintensity values and a second pixel position corresponding to a minimaof the difference between the first and second sets of pixel intensityvalues, wherein the first pixel position is representative of one of thefirst sensor element position and the second sensor element position andthe second position corresponds to the other of the first sensor elementposition and the second sensor element position.
 5. The method of claim4, further comprising: using one or both of the first and second pixelpositions to determine a rate of flow of the fluid in the flow controlsystem.
 6. The method of claim 4, further comprising: calculating one orboth of an axial distance between the first pixel position and thesecond pixel position and a rate of change of position of the sensorelement between the first sensor element position and the second sensorelement position.
 7. The method of claim 6, further comprising:correlating one or both of said axial distance between the first pixelposition and the second pixel position and said rate of change ofposition of the sensor element between the first sensor element positionand the second sensor element position with a viscosity.
 8. The methodof claim 7, wherein said correlating is performed by comparing one orboth of said axial distance between the first pixel position and thesecond pixel position and said rate of change of position of the sensorelement between the first sensor element position and the second sensorelement position with known sensor element behavior for one or morefluids of known viscosity, said known sensor element behavior associatedwith said known viscosity.
 9. The method of claim 7, further comprising:inputting fluid identifying information; determining an expectedviscosity based on said fluid identifying information; comparing saidviscosity determined by said correlating with said expected viscosity;if said viscosity is not within some predetermined threshold of saidexpected viscosity, outputting an indication of an error condition. 10.The method of claim 1, wherein the abrupt change in the fluid drivingpressure is an abrupt increase in the fluid driving pressure.
 11. Anoptical position measurement apparatus for determining a position of amovable flow sensor element of a flow sensor in a flow control system,the flow sensor being of a type having an axially-extending flowpassageway for a liquid to be delivered by the flow control system, themovable sensor element being axially movable within the flow passagewayin response to a first force exerted on the sensor element by the liquidas it passes through the flow passageway and in response to a secondforce exerted on the sensor element in a direction opposite the firstforce by a spring received within the flow passageway, the sensorreaching equilibrium position within the flow passageway when themagnitudes of the first and second forces are equal, the equilibriumposition corresponding to a axial position within the flow passagewaywhich varies with varying flow rate, the flow control system being of atype having a source of fluid driving pressure for driving the fluidthrough the flow passageway, the flow rate being responsive tovariations in the fluid driving pressure, the apparatus comprising: alight source and a photosensitive pixel array for illumination with saidlight source, the photosensitive pixel array extending axially along theflow passageway and the sensor element disposed in an optical pathbetween the light source and the photosensitive pixel array; a processorreceiving signals from said photosensitive pixel array to generate afirst set of pixel intensity values for the photosensitive pixel array,the first set of pixel intensity values representative of a first sensorelement position corresponding to a first flow rate and a second set ofpixel intensity values for the photosensitive pixel array, the secondset of pixel intensity values representative of a second sensor elementposition corresponding to a second flow rate; a source of pressurizedair for introducing an abrupt change in the fluid driving pressure; saidprocessor calculating a difference between the first and second sets ofpixel intensity values as a function of pixel position.
 12. Theapparatus of claim 11, wherein the first and second sets of pixelintensity values are values representative of pixel voltages.
 13. Theapparatus of claim 11, wherein the sensor element is an opticallytransmissive ball.
 14. The apparatus of claim 11, further comprising:said processor identifying a first pixel position corresponding to amaxima of the difference between the first and second sets of pixelintensity values and a second pixel position corresponding to a minimaof the difference between the first and second sets of pixel intensityvalues, wherein the first pixel position is representative of one of thefirst sensor element position and the second sensor element position andthe second position corresponds to the other of the first sensor elementposition and the second sensor element position.
 15. The apparatus ofclaim 14, further comprising: said processor using one or both of thefirst and second pixel positions to determine a rate of flow of thefluid in the flow control system.
 16. The apparatus of claim 14, furthercomprising: said processor calculating one or both of an axial distancebetween the first pixel position and the second pixel position and arate of change of position of the sensor element between the firstsensor element position and the second sensor element position.
 17. Theapparatus of claim 16, further comprising: a database storing knownsensor element behavior for one or more fluids of known viscosity andcorrelating said known sensor element behavior with viscosity.
 18. Theapparatus of claim 16, further comprising: said processor determiningviscosity by comparing one or both of said axial distance between thefirst pixel position and the second pixel position and said rate ofchange of position of the sensor element between the first sensorelement position and the second sensor element position with said knownsensor element behavior.